Simulation of biomass particle evolution under pyrolysis conditions using lattice Boltzmann method

Abstract

To accurately characterize biomass fast pyrolysis at the particle scale, intra-particle transport phenomena needs to be considered together with the gas flow surrounding the particle. In this study, a detailed numerical method was used to simulate the evolution of biomass particles under fast pyrolysis conditions. To conduct particle-scale simulations, the lattice Boltzmann method (LBM) was employed to solve the conservation equations, and pyrolysis kinetics were implemented to describe the chemical reactions. The present model was validated by comparing the numerical results with experimental data for a single biomass particle under pyrolysis conditions. The predicted temperature and conversion history agree with the experimental data. The temperature and density fields in the particle were found to be anisotropic due to the effect of the gas flow surrounding the particle, which was ignored by the 1D model. The non-uniform distributions of surface temperature indicate that using a constant temperature or heat flux as boundary conditions may cause numerical errors. Sensitivity analysis shows that density is the most influential parameter while porosity is the least. The heat of reaction converting the intermediate solid to char is more dominant than those of the three primary reactions. A parametric study was conducted to investigate the effect of particle shape on conversion time and final product yields. The simulation results show that the conversion time decreased when using the elliptic particle instead of the regular (circular) particle. The model also shows that more tar and syngas were produced, while less char was generated from an elliptic particle. The effect of particle shape on the center temperature of the biomass particle can be explained by comparing the heat transfer conditions at the front of the particle and the convective heat transfer at the top and bottom of the particle. The results demonstrated that the current LBM framework has the ability to reveal the detailed conversion process of biomass particles under various pyrolysis conditions, which can then be used to improve engineering models for reactor-scale simulation.